This invention generally relates to ultrasound imaging systems. In particular, the invention relates to methods and apparatus for imaging blood flow and contrast agents.
Conventional ultrasound transducers for use in medical diagnostic imaging transmit a broadband signal centered at a fundamental frequency ƒ0, which is applied separately to each transducer element making up the transmit aperture by a respective pulser. The pulsers are activated with time delays that produce the desired focusing of a transmit beam at a particular transmit focal position. As the transmit beam propagates through tissue, echoes are created when the ultrasound wave is scattered or reflected off of the boundaries between regions of different density. The transducer array is used to transduce these ultrasound echoes into electrical signals, which are processed to produce an image of the tissue. These ultrasound images are formed from a combination of fundamental (linear) and harmonic (nonlinear) signal components, the latter of which are generated in nonlinear media such as tissue or a blood stream containing contrast agents. With scattering of linear signals, the received signal is a time-shifted, amplitude-scaled version of the transmitted signal. This is not true for acoustic media which scatter nonlinear ultrasound waves.
The echoes from a high-level signal transmission will contain both linear and nonlinear signal components. In certain instances ultrasound images may be improved by suppressing the fundamental and emphasizing the harmonic (nonlinear) signal components. If the transmitted center frequency is at ƒ0, then tissue/contrast nonlinearities will generate harmonics at kƒ0 and subharmonics at ƒ0/k, where k is an integer greater than or equal to 2. Imaging of harmonic signals has been performed by transmitting a narrow-band signal at frequency ƒ0 and receiving at a band centered at frequency 2ƒ0 (second harmonic) followed by receive signal processing.
One fundamental problem faced by harmonic imaging is low harmonic-to-noise ratio (HNR) since the harmonic signals are at least an order of magnitude lower in amplitude than the fundamental signal. A secondary problem is insufficient isolation of the harmonic signal from the fundamental as measured by a low harmonic-to-fundamental ratio (HFR).
Coded Excitation is the transmission of long encoded pulse sequences and decoding of the received signals in order to improve image SNR and/or resolution. The energy contained in a long transmit pulse sequence is compressed into a short time interval on receive by virtue of the code. Coded excitation is a well-known technique in medical ultrasound imaging. For example, the use of Golay codes is disclosed in U.S. Pat. No. 5,984,869 issued on Nov. 16, 1999.
Likewise the technique of harmonic imaging using contrast agents is known.
Harmonic imaging images the nonlinear signal components produced inside the body that is used to enhance contrast agent signal when imaging blood flow. The technique of harmonic imaging using contrast agents is presented in de Jong et al., xe2x80x9cCharacteristics of Contrast Agents and 2D Imaging,xe2x80x9d Proc. 1996 IEEE Intl Ultrasonics Symp., pp. 1449-1458 (1997). Contrast harmonics can greatly improve vascular studies.
Harmonic imaging that uses two transmits with 180-degree phase shifts has been disclosed. The pulse inversion between the two transmits suppresses the fundamental signal and leaves the harmonic signal to form the image. Harmonic coded excitation that uses pulse sequences with 0 and 90-degree phase symbols (e.g., xe2x80x9c1xe2x80x9d and xe2x80x9cjxe2x80x9d, where j2=xe2x88x921) has been disclosed by Takeuchi in xe2x80x9cCoded Excitation for Harmonic Imaging,xe2x80x9d Proc. 1996 EEE Intl Ultrasonics Symp., pp. 1433-1436 (1997) and by Chiao et al. in U.S. patent application Ser. No. 09/494,465 filed on Jan. 31, 2000. However, a method to suppress the fundamental signal on reception was not specified in those disclosures. Harmonic coded excitation using Quadrature Phase Shift Keying (QPSK) (i.e., symbols 1, xe2x88x921, j and xe2x88x92j) with suppression of the fundamental signal on reception was disclosed in U.S. Pat. No. 6,050,947 issued on Apr. 18, 2000.
It is known to use ultrasound imaging systems to image perfusion in an anatomical region. Perfusion imaging provides an assessment of rate of blood flow in the anatomical region and is useful for diagnosis of pathological conditions such as myocardial ischemia, tumor diagnosis, and transplant evaluations. Because much of perfusion occurs in micro-capillaries, perfusion imaging requires the ability to distinguish low-speed flow from tissue. The problem is exacerbated by the small diameter of the capillaries which limits the echo strength from the blood and thus the signal-to-noise ratio (SNR). Conventional flow imaging that uses color or power Doppler is unable to surmount these problems and is ineffective for perfusion imaging.
Recent advances in contrast agents have opened the door to perfusion imaging. Contrast agents are typically encapsulated gas micro-bubbles between 0.1 micron and 10 microns in diameter. When introduced into the body by injection, contrast agents serve as high-reflectivity markers for blood flow and perfusion. Ultrasound energy incident on the micro-bubbles is strongly reflected at the incident (fundamental) frequencies and at the harmonic frequencies. Because the ratio of contrast agent-generated signal to tissue-generated signal is much larger for harmonic signal components than for the fundamental signal components, the harmonic signal components are preferred for perfusion imaging.
There are several prior art methods for perfusion imaging. Methods have been investigated that destroy bubbles in feeding vessels in such a way that a near impulse input, which corresponds to a nearly perfect bolus, is generated. Tracking the passage of the impulse through a tissue region gives the tissue response function from which perfusion can be calculated. Another potential method employs speckle decorrelation to estimate the local mean transit time through tissue. The mean transit time along with an estimate of the fractional blood volume would produce an estimate of perfusion. In a third prior art technique, the sampled perfusion method, a sequence of bursting and imaging pulses are transmitted with varying time intervals. The bursting pulses destroy the contrast micro-bubbles within the region of interest while the imaging pulses measure the amount of contrast agent refill that has occurred over that time interval. The perfusion curve over time is measured by using multiple pairs of bursting and imaging pulses with varying intervals. The drawback of this method is that the perfusion curve is built up slowly and extra contrast agent may be required as contrast bubbles are repeatedly cleared and refilled within the region of interest. Finally, a continuous perfusion method uses low-mechanical-index (i.e., low-amplitude) imaging pulses that do not burst contrast bubbles. In this method, the bursting pulse is used only once, followed by imaging pulses that measure the refill at each time instant. This method provides real-time perfusion imaging, but suffers from low SNR because of the low-mechanical-index imaging pulses that are used.
There is a need for a method of real-time imaging of perfusion in an anatomical region that provides improved SNR.
The present invention is directed to a method and an apparatus for providing perfusion images having improved SNR using coded excitation in conjunction with the continuous perfusion method. Coded excitation is the transmission of long encoded pulse trains and decoding of the received signals in order to improve image SNR. Specifically, the energy contained in the long transmit pulse train is compressed into a short time interval on receive by virtue of the code.
The method in accordance with the preferred embodiments of the invention comprises the steps of bursting contrast micro-bubbles (using either a very large bursting pulse or by scanning a smaller bursting pulse) in the region of interest in one or more frames followed by scanning one or more encoded imaging pulses in each subsequent frame. The bursting pulse (or pulses) is intended to break contrast micro-bubbles within a region and therefore should have high mechanical index and low frequency. The basic concept is to use a very low-amplitude, focused, encoded pulse train (hereinafter xe2x80x9cimaging pulsesxe2x80x9d) to image contrast agents. The low amplitude prevents the contrast bubbles in the transmit focal zone from being destroyed while imaging, and the coded excitation provides the necessary SNR. The imaging pulses are transmitted during refilling of the transmit focal zone with contrast agent subsequent to transmission of the bursting pulse into the same zone.
In accordance with the preferred embodiments, QPSK-encoded imaging pulses centered at a fundamental frequency and focused in the transmit focal zone are transmitted. In a one-transmit embodiment, the imaging pulse waveform is encoded such that the received second harmonic signal will be encoded with a Barker code. In a two-transmit embodiment, the imaging pulse waveforms are encoded such that the respective received second harmonic signals are encoded with respective Golay codes of a Golay code pair. A Golay code pair X and Y satisfies the complementarity property X*X+Y*Y=xcex4(n), where the xe2x80x9c*xe2x80x9d symbol denotes convolution and xcex4(n) is a Kronecker delta function. In a four-transmit embodiment, a different QPSK transmit code is used for each of four transmits A, B, C and D. The transmit codes are selected such that (Axe2x88x92B) and (Cxe2x88x92D) are encoded by Y and xe2x88x92X, respectively, while (A2xe2x88x92B2) and (C2xe2x88x92D2) are encoded by X and Y, respectively, where X and Y form a Golay code pair. X is a sequence such that X=x(n), n=0, 1, 2, . . . , (Nxe2x88x921), and X denotes the reversal of X given by X=x(Nxe2x88x921xe2x88x92n) for n=0, 1, 2, . . . , (Nxe2x88x921). The same is true for Y and Y.
In the two- and four-transmit embodiments, the encoded pulses (transmitted at different times) are focused at the same transmit focal position. This is repeated for each transmit focal position in the scanning plane. The resulting receive vectors are decoded to form a compressed pulse vector. The fundamental signal components are suppressed while the second harmonic signal components are fully compressed in the decoded waveform. The evolution of brightness at registered points in a sequence of image frames indicates the change in contrast agent concentration or perfusion at those points.